Abstract: Last month's Technical Tidbit
presented simulation results in the time domain showing how probing a signal
can significantly effect it. This month, simulation data is presented in
the frequency domain to again show that under realistic conditions, active
probes without damping resistors can significantly affect the signal to be
measured. As before, the result can be more serious than just signal loading.

Discussion: Figure 1 shows simulation results of probe response (peaked
signal in blue) and the actual voltage at the measured node (red with dip)
for a 1 Volt source having an impedance of 25 Ohms in series with 5
nH without a probe damping resistor (a series resistor ~100-200 Ohms at the probe tip). The frequency range of 100 MHz to just
under 5 GHz is displayed. The source resistance was chosen to represent a
gate output resistance and the inductance to represent nominal package inductance.
Many modern chip packages have signal path inductance ranging from 2 to 8 nH*.

The probe model used in the spice simulation is a manufacturer's multi-element
model of a 4 GHz active probe fitted with a 5 cm extension adapter that included
a damping resistor at the tip of about 200 Ohms. The simulation modeled the
probe's connections and input circuitry, but not the probe amplifier response
for simplicity. For the plots in Figure 1, the damping resistor was removed.
Without the damping resistor, the probe's input impedance and response is
similar to active probe designs that do not include a damping resistor. The
probe response has a peaked response that is common with even
short probe connections on many active probes. In this case the probe response has a 14 dB gain peak at about 800 MHz!
The probe response in Figure 1 agrees well with measured results on active
probes without damping resistors, thus helping to verify the simulation results.

However, the important detail in Figure 1 is the actual signal on the
node during the measurement, the red trace with a dip at 900 MHz. With the
25 Ohm + 5nH signal source used in the simulation, the node voltage fell
about 9 dB at 900 MHz, low enough to possibly cause signal integrity
problems. And, this happened while the probe was displaying the signal with about
double the original amplitude with no probe connected. Both the probe gain peak and the dip in the
measured node voltage are sensitive to the amount of source inductance, but
even a few nH of inductance can be a problem. Of course, the node voltage
cannot be observed on a real circuit because it is changed by probing it,
but the simulation suggests a real possibility of problems caused by an active
probe without a damping resistor at the tip.

Figure 2 shows the probe response (blue trace falling off at high frequencies)
and node voltage (red trace gradually increasing at high frequencies) when
the ~200 Ohm damping resistor was present in the circuit. The probe response
is smooth with a well behaved rolloff. Note the inductance of the 5 cm extension
causes the 3dB rolloff point to be only about 1.2 GHz rather than the 4 GHz
bandwidth the probe actually is capable of. Although the damping resistor
can smooth the probe response as well as reduce effects in the measured circuit,
it cannot restore bandwidth lost due to inductance in the probe connections.
Probe connections should be kept as short as possible for this reason and
other reasons. That being said, the frequency responses in Figure 2 are very
good compared to those in Figure 1. Only passive probes can achieve better
performance in terms of tolerance to probe connection inductance.

Summary and Conclusion: Active probes, or any type of scope probe
for that matter, can have significant effects on the measured signal that
are quite distinct from the response of the probe itself as evidenced by
the frequency domain plots above. Active probes should always have damping
resistance either built
in or added and probe connections should be kept as short as possible.

MI-SUGAR circuit simulation program version 0.4.3 for Mac OS X (Spice with a graphical interface, free!) Click here
to download the program for Mac OS X - not available for Windows. If you
would like more information on MI-SUGAR, click here to send email to Berk Ozer, the program's author.